Garden Calendar: Wisteria provides late spring color

Just as the lilac blossoms (and questions about them) fade, the brilliant purple and white panicles of wisteria come along to delight their owners and passers-by.

Wisteria is a woody vine whose flowers hang in clusters that reach 12 to 18 inches long on some varieties. They are often seen growing over arbors and gazebos, with their flowers hanging down through the spaces in the structure like clusters of grapes.

There are three species of wisteria commonly available.

Chinese wisteria, Wisteria sinensis, may be the most popular because of flowers that open all at once for a more showy display than the other wisterias.

Japanese wisteria, Wisteria floribunda, offers a wider range of flower colors and blooms a little later than the Chinese variety.

American wisteria, Wisteria frutescens, is less showy and fragrant than its cousins but is also less aggressive. American wisteria also blooms later — typically in June instead of early-to-late May like the others.

I hope the word aggressive catches some attention, because the Chinese and Japanese varieties usually fit that description well.

Experienced gardeners know to be wary of aggressive plants as well as descriptors like vigorous, extremely hardy, easy to grow and fast-growing. A gardener who wants to grow wisteria should plan on spending enough time with the plant each year to keep it in check.

Besides their aggressiveness, wisterias have one other somewhat major difficulty: There are a finicky few that fail to or take a long time to bloom.

“If a wisteria isn’t blooming, it probably isn’t getting enough sunlight or it could have possibly gotten cold enough in the winter to kill the flower buds,” says Dr. Jason Griffin, horticulture extension specialist with K-State Research and Extension. Griffin specialized in tree, shrub and other woody plant research and is director of the John C. Pair Horticulture Research Center in Haysville.

As with many plants, excessive nitrogen fertilization or pruning at the wrong time could also inhibit blooming. In wisteria’s case, avoid pruning in winter and early spring.

I have heard some horror stories, however, from gardeners who seem to be treating their wisteria just right and still are not getting any blossoms.

Root pruning in the fall is recommended by many gardening resources but is not always effective.

If nonblooming wisterias do not fall under one of the possibilities mentioned before, they are most likely still in the juvenile stage. For grafted varieties, the juvenile stage of the plant typically lasts three to four years, but wisteria grown from seed can take 10 to 15 years to mature into flower production.

Local landscape architect Reed Dillon says he prefers to purchase wisteria already in bloom to avoid any nonflowering problems later.

If you find a good wisteria and are planting one for the first time, plan accordingly for the plant’s size and aggressiveness. The vines can grow to 25 feet or more and can girdle or smother other plants. Griffin recommends that gardeners growing wisteria prune every year to keep it in check.

“Enjoy the blooms and the fragrance, then cut it back,” he advises. “No doubt about it, it will spread.”

Although often pictured and seen growing over arbors and gazebos, Griffin notes: “I never recommend planting it next to a structure unless you really want to replace that over time or you can keep the plant really pruned down.” Wisteria can be pruned into a tree or shrub form.

American wisteria is considered native to the eastern part of the United States and as far west as Missouri and Oklahoma.

The American species is considered to flower more reliably than the Chinese and Japanese species and is typically less aggressive. “Amethyst Falls” is a popular variety.

Griffin says that even though the blossoms are smaller on American wisteria, plants will sometimes offer a second, albeit subdued, bloom during the same season.